Method and apparatus for dynamically aligning a printer printhead

ABSTRACT

In order to dynamically align one or more printheads in a printer, a referencing mechanism is placed on the printer and a detector is placed on the printhead. The printhead is moved past two spaced apart reference indicia of the referencing mechanism. The passing of a first of the spaced apart reference indicia is detected and the passing of a second of the spaced apart reference indicia is detected. The time between the detection of the first reference indicia passage and the detection of the second reference indicia passage is determined and a delay time, related to the measured period of time, is created. Energization of an ink drop ejection is delayed for the duration of the delay time.

The present invention is a continuation-in-part of U.S. patentapplication Ser. No. 08/537,223 filed on Sep. 29, 1995 is now U.S. Pat.No. 5,751,305 and assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION

The present invention relates generally to a method and apparatus foraligning printing mechanisms and more particularly a method andapparatus for aligning multiple printheads or print cartridges in an inkdroplet ejection printer such as a thermal inkjet printer.

One conventional type of printer is one which forms characters andimages on a medium, such as paper, by expelling droplets of ink in acontrolled fashion so that the droplets land on the medium. Such aprinter can be conceptualized as a mechanism for moving and placing themedium in a position such that the ink droplets can be placed on themedium, a printing cartridge which controls the flow of ink and expelsdroplets of ink to the medium, and appropriate control hardware andsoftware. A conventional print cartridge for an inkjet type printercomprises an ink containment device and a fingernail-sized apparatus,commonly known as a printhead, which heats and expels ink droplets in acontrolled fashion. Typically, the printhead is a laminate structureincluding a semiconductor base, a barrier material structure which ishoneycombed with ink flow channels, and an orifice plate which isperforated with holes or orifices with diameters smaller than a humanhair and arranged in a pattern which allows ink droplets to be expelledin a controlled pattern. In an inkjet printer the heating and expulsionmechanism consists of a plurality of heater resistors formed in thesemiconductor substrate and associated with an ink chamber formed in thebarrier layer and one of the orifices in the orifice plate. Each of theheater resistors is connected to the controlling mechanism of theprinter such that each of the resistors may be independently energizedto quickly vaporize to expel a droplet of ink.

In some applications, more than one inkjet print cartridge will bedesigned into a printer. Usually this multiple print cartridge assemblyis created to accommodate multiple colors of ink. Properly controllingthe arrangement of various droplets of ink of different colors willresult in a wide spectrum of perceivable colors. The clarity and qualityof the resultant image is affected by the accuracy of the placement ofthe ink droplets on the medium. Printers which use multiple printcartridges to cooperatively form a single image usually requiremechanical or electronic adjustment so that ink droplets printed by onecartridge alight at precise locations on the receiving medium relativeto those printed by another cartridge in the printer.

Cartridge-to-cartridge alignment has been eliminated in some printerswith the use of a single multi-color ink cartridge having a printheademploying three sets of orifices arranged in a group and receiving onecolor of ink for each group on the printhead. Such a single multi-colorprint cartridge is inherently self-aligning due to the precisepositioning of one set of orifices relative to another on the singleorifice plate on the multi-color print cartridge. Even for thiscartridge, however, unless other compensation is made, the orifice plateof the printhead should be oriented precisely perpendicular to thedirection of travel for accurately printed results.

Mechanical alignment of print cartridges is simple but expensive,requiring precision features created in the orifice plate of theprinthead, precision alignment of the cartridges during manufacture toalignment structures or secondary milling of alignment structures oradjustment within the printers cartridge carriage. In each of theseforegoing implementations, there are stringent requirements on theprinter and the cartridge carriage for either precision duringmanufacture and long term stability, or complex adjustability and humanintervention. Electronic alignment typically requires printing inkdroplet dots on a separate region of the medium, scanning the mediumwith a detector for these dots, then establishing time delays within theprinter to compensate for the measured offsets. Again, printercomplexity or human intervention and judgment are required to optimizethis form of alignment.

Each of the foregoing techniques do not dynamically compensate formovement of the print cartridge within the carriage between alignmentcycles due to thermal expansion or wear or loosening within themechanism. Each of these methods add mechanical or electronic complexityto the printer. Thus, a need exists for a method and apparatus whichreadily adjusts for horizontal cartridge-to-cartridge alignment errorsin a multiple cartridge printer. Furthermore, vertical and rotationaloffsets also need compensation to precisely align the ink droplets onthe media.

SUMMARY OF THE INVENTION

A printer dynamically aligns at least two printheads relative to areferencing mechanism. The printer moves the printhead past at least tworeference indicia having a predetermined spacing. The passing of a firstof the at least two reference indicia and the passing of a second of theat least two reference indicia is detected. The period of time betweenthe detection of the first reference indicia passage and the detectionof the second reference indicia passage is determined and a delay time,related to the measured period of time, is created. The energization ofan ink drop ejection apparatus is consequently delayed for the durationof the delay time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of a printer which may employ the presentinvention.

FIG. 2 is an isometric view of a print cartridge which may be used inthe printer of FIG. 1 and which may employ the present invention.

FIG. 3 is an isometric view of a print cartridge carriage for theprinter of FIG. 1 and print cartridges of FIG. 2 and which may employthe present invention.

FIG. 4 is a planar view of a printhead for a print cartridge which mayemploy the present invention.

FIG. 5 is a cross sectioned view of the printhead of FIG. 4 alongsection A—A.

FIG. 6 is a simplified view of the orientation of the substrate, orificeplate, and alignment plate which may be employed in the presentinvention.

FIG. 7 is a timing diagram of electrical signals which may be producedby the printhead of FIG. 6.

FIG. 8 is a schematic diagram of the connection of the connection ofhorizontal, vertical and rotational correction circuits which may beemployed in the present invention.

FIG. 9 is a schematic diagram of a horizontal error correction circuitwhich may be employed in the present invention.

FIG. 10 is a schematic diagram of the vertical error correction circuitwhich may be employed in the present invention.

FIG. 11 is a schematic diagram of the rotational error correctioncircuit which may be employed in the present invention for heaterresistors which are associated with orifices furthes from rotationaldatum.

FIG. 12 is a schematic diagram of a rotational error correction circuitwhich may be employed in the present invention for heater resistorswhich are associated with orifices of intermediate distance from therotational datum.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention encompasses a method and apparatus forself-aligning one or more print cartridges in a dynamic fashion in aprinter. An alignment pattern having a large light-to-dark ratio isplaced at a functional location within the printer but away from themedium. Photosensitive devices, created as part of the semiconductorsubstrate and associated with the orifice plate, are arranged to readthe high contrast features of the alignment plate. Timing is derivedfrom a clocking pulse and an alignment pulse provided by the printer totime the printheads location relative to the alignment feature on theprinter. Firing of the heater resistors are then delayed or advanced asdetermined by the timing and is implemented by the shift registersconstructed, in the preferred embodiment, on the semiconductor substrateof the printhead.

A simplified diagram of a printer is shown in FIG. 1. Medium 101 ismoved past print cartridges 103 and 105 in a direction arbitrarilydesignated the “Y” direction (into the plane of paper of FIG. 1) by aplaten motor 109. The print cartridges 103 and 105 are mounted in acartridge carrier 107 and are scanned back and forth across the mediumin an orthogonal (“X”) direction by a carriage motor 111. The platenmotor 109 and the carriage motor 111 are conventionally under thecontrol of a media and cartridge position controller 113 suchpositioning and control apparatus are known and are further described inU.S. Pat. No. 5,070,410. The printhead carriage position may also be the“X” direction for printing by the use of a linear encoder strip such asthat disclosed in U.S. Pat. No. 5,297,017. In one embodiment, a printpositioning device 307 (FIG. 3) is mounted on the cartridge carrier 107to detect print positioning indicia disposed on a linear encoder strip309. Thus the medium is positioned in a location so that the printcartridges 103 and 105 may to eject droplets of ink as required by thedata which is input to the droplet firing controller 115 of the printerin a band parallel to the “X” direction as the print cartridges 103 and105 are scanned across the medium by the carriage motor 111. When theprint cartridge 103 and 105 reach the end of their travel at an edge ofthe medium 101, the medium 101 typically is incrementally advanced bythe media position control 113 and platen motor 109, and the printcartridge 103 and 105 are returned along the “X” axis while printinganother band of ink droplet dots on the medium 101 until the oppositeend of the medium is reached. From time to time, the print cartridges103 and 105 may be moved away from the medium 101 and aligned with aservice station 117 so that the printheads of the cartridges 103, 105may be wiped clean of debris and the orifices purged of any materialwhich may obstruct the ejection of ink droplets. A service station whichmay be employed in the preferred embodiment of the present invention isfurther described in U.S. Pat. No. 5,103,244. In the present invention,an alignment reference indicia be located on the service station 117 toprovide a location reference for each of the print cartridges 103 and105 thereby relating each cartridge to one another and the cartridgecarrier.

A print cartridge which may be used in the present invention is shown inFIG. 2. Generally, a majority of the volume of the print cartridge isdedicated to the containment of ink. At one end of the cartridge aprinthead 203 is affixed to the print cartridge and internally coupledto the ink supply within the ink cartridge. Electrical connections aremade to the heater resistors within the printhead 203 by a flexiblecircuit 205. The flexible circuit 205 also mates with associatedelectrical connectors of the print carriage of the printer. When aplurality of print cartridges are mounted in a printer, they arearranged in a side-by-side carriage configuration for the preferredembodiment as shown in FIG. 3. Electrical connection to the printcartridges are made via mating connectors (not shown) disposed on theprint carriage 107. The printhead of each cartridge is typicallyoriented down relative to the direction of gravity and positioned overthe media upon which ink is to be printed. The service station 117 isalso shown in greater detail with the print cartridges 103, 105positioned over a portion 301 of the service station 117 which caps theprintheads and prevents ink from drying in the orifices when positionedin contact with the printheads. A wiper mechanism 303 is arranged in adirection from the capping mechanism such that when the print carriagereturns from the medium being printed, the printheads are first wiped bythe wiper mechanism 303 and then capped by the capping portion 301. Itis a feature of the present invention that an alignment plate 305 bedisposed within the fixed portion of the printer and that, in thepreferred embodiment, the alignment plate 305 is placed on the servicestation. The service station also provides power for the light source(not shown) beneath the alignment plate 305.

A magnified planar view of the printhead is shown in FIG. 4. A pluralityof orifices in two columns 403, 405 are depicted in the orifice plate ofthe printhead 203. Although shown in two collinear columns, the orificesmay be staggered in the +X or −X direction from the general line of eachcolumn. Such stagger amount is known and electronically compensated forwithin the control of the printer. In the preferred embodiment, a totalof 54 orifices are employed in equal division in the two columns of theorifice plate. At separate points of the orifice plate but created bythe same process which creates each orifice, two alignment orifices 407and 409 are found in the orifice plate of the printhead 203. In thepreferred embodiment alignment aperture 407 and alignment aperture 409are placed diagonally across the surface of the orifice plate as shown.Alternatively, an alignment apeture 411 may be placed on an imaginaryline with apeture 407 which is parallel to the “Y” direction.

A cross section of the printhead 203 along the section line AA is shownin FIG. 5. Features relating to structure which supports the filling ofink have been deleted from FIG. 5 for clarity. A semiconductor substrate501 is conventionally processed to include heater resistors 503 and 505which are appropriately connected by electrical connectors 507, 509,511, and 513. Ink firing chambers, which in operation are filled withink, are defined by the substrate, the barrier layer material 515 andorifice plate 517. The orifice plate 517 overlays the barrier material515 such that the orifices 403 and 405 are arranged in association withthe heater resistors 503 and 505 and the formed ink firing chambers. Aspart of the processing steps of the semiconductor material 501 for thepreferred embodiment, a photosensitive area 519 is created usingconventional photolithographic defined semiconductor processes. Thisphotosensitive area 519 is then connected by way of conductors 521 and523 to appropriate parts in the electronic circuit to be describedlater. The alignment aperture 407 is positioned relative tophotosensitive area 519 such that light falling perpendicularly to thesurface of the orifice plate will fall on the photosensitive area 519.It is a feature of the present invention that the alignment orifice 407is produced in the same process as the ink firing orifices 403 and 405thereby providing nearly perfect registration between the orifices andthe alignment orifice. Furthermore, since firing resistors 503, 505, andphotosensitive area 519 are all produced by precision semiconductorphotolithographic techniques, they too, are precisely aligned.

Alignment and operation of the alignment plate 305 and the printhead canbe apprehended from the drawing of FIG. 6. The service station 117 ofFIG. 3 has been omitted for clarity, leaving only the alignment plate305 to illustrate the alignment technique employed in the preferredembodiment. A light source 601 is disposed within the service station117 and arranged in a fashion such that the light falls perpendicular tothe plane of the alignment plate 305. In the preferred embodiment, thisis accomplished by using a conventional lensed light emitting diode butany source of essentially parallel light rays may be employed withoutdeparting from the spirit of the present invention.

Two opaque stripes 603 and 605 are formed in the light transmitting(translucent or transparent) alignment plate 305 and cause respectiveshadows 607, 609 to fall upon the orifice plate 517 of the printhead. Inthe preferred embodiment, the alignment plate 305 is made of convential,optically translucent plastic having stripes 603 and 605 conventionallyetched and printed into the surface of the alignment plate plastic insuch a manner that the width of the stripes is at least as wide as thediameter of an alignment aperature. As the orifice plate 517 passes infront of the alignment plate 305 during a trip to the service station oras otherwise required, the shadows 607, 609 pass over the alignmentorifices 407, 409. As illustrated in FIG. 5, the shadow 607 occludes thelight falling upon photosensitive area 519 thereby generating anelectrical signal which will be described later. In the preferredembodiment, the spacing between the alignment plate 305 of the servicestation 117 and the orifice plate 517 is 2 mm but this spacing is notcritical as long as the light emanating through the alignment plate isparallel.

In an alternative embodiment, the alignment plate 305 may be made opaquewith transparent slits to emit light. In either embodiment, the presenceand/or absence of light passing through apertures in the orifice plateand falling upon photo receptors in the printhead is used to indicatelocation of the printhead.

After the photoreceptor signal is processed and shaped, electricalsignals generated by the photosensitive areas are shown in the timingdiagram of FIG. 7. The electrical output signal from the photosensitivearea beneath the alignment orifice 407 is illustrated as output 701 andthe electrical output signal from the photosensitive area beneath thealignment orifice 409 is illustrated as output 703. In the preferredembodiment, the printer provides a reference indicia synchronizingsignal 705 which is used to indicate the printing position of thecartridge carrier 107 relative to the printer. This signal may begenerated in a number of conventional ways; in the preferred embodiment,the cartridge carrier position is detected by way of indicia marks on alinear encoder strip 309. It should be noted that other techniques ofposition location, such as stepper motor steps or light interferometry,may also be effectively used to determine printing location of thecartridge carrier 107. An indicia detector is located in the printpositioning device 307 and provides a series of electrical pulses to theprinter position controller 113 thereby indicating the crossing ofindicia marks. A particular reference synchronizing pulse 707 of theelectrical pulses is used to trigger the alignment process. Subsequentto pulse 707, the indicia detector electrical pulses of synchronizingsignal 705 are used as timing pulses for printhead alignment. Asillustrated in FIG. 7, the reference pulse 707 occurs at time t₁. As thealignment orifice 407 of the first cartridge is moved past the opaquestrip 605, the light falling upon the photosensitive area 519 isinterrupted as the opaque strip 605 is passed and an electrical signal,represented by pulse 709 in FIG. 7, is generated. Pulse 709 isdesignated as occurring a particular number of timing pulses from t1 andequivalent to a time t₂. As the first print cartridge continues to move,opaque strip 603 throws a shadow across alignment orifice 407 and thephotosensitive area 519 and an electrical signal, represented by pulse711 in FIG. 7, is generated and this pulse is designated as occurringfollowing a number of timing pulses—at time t₃. Evaluation of thedifference in the number of timing pulses (i.e. the time) between t₁ andt₂ yields an offset or position (“X”) indication for the cartridgerelative to the index position. Since the opaque strip 603 is orientedat an angle relative to opaque strip 605 evaluation of the difference intime between t₃ and t₂ from an expected time (related to the number ofexpected timing pulses and the distance between the opaque strips 603and 605 at the correct “Y” elevation of the print cartridge) yields anindication of a position error in the “Y” direction. The absolute time(or number of pulses) is not a critical parameter in practicing theinvention so long as the difference is consistent.

Referring now to the output signal 703, it can be seen that pulses 713and 715, similar to pulses 709 and 711, are generated by thephotosensitive area associated with alignment aperture 409. Since theopaque strips 603 and 605 are further apart at the lower end of theprinthead than they are at the upper end of the printhead, the leadingedges of pulses 713 and 715 (at times t₄ and t₅, respectively, thenumber of timing pulses being equated to time) are further apart in time(t₅−t₄) than the pulses 709 and 711. If the print cartridge has arotational (θ) error in its orientation relative to the printer there isat least a difference in the detected time of pulses 709 and 713. Forexample, if the expected leading edge time for pulses 709 and 713 tooccur were t₂ and t₄ but the actual time detected for pulse 713 occurredat a later times t₄, the print cartridge has a rotational position errorin the −θ direction. Likewise if the actual pulse 713 time detectionpreceeded the expected time, the print cartridge has a rotationalposition in the +θ direction. In a two-or multi-print cartridge printer,each of the print cartridges would undergo the preceeding measurement ofposition error relative to a fixed reference position on the printer.

In the preferred embodiment, correction of horizontal, vertical, androtational position errors of the cartridge in made by sequentiallyadding an appropriate delay in a heater risistor firing pulse outputfrom the droplet firing controller 115. This process can be preceivedfrom the block diagram of FIG. 8. A pulse 800 of electric energy isoutput from droplet firing controller 115 and applied to a horizontalposition correction circuit 801 for delay, if necessary. The delayed (ifneeded) firing pulse 802 is coupled to a vertical position correctioncircuit 803 for delay if necessary for correction of a vertical positionerror.

The delayed (if needed) firing pulse 804 is coupled to a rotationalerror correction circuit 805 for appropriate delay to correct forrotational errors. A final heater resistor firing pulse 806 is thenoutput to the heater resistor 809 to energize the resistor, heat andvaporize the ink, and expel a droplet of ink for prining on the medium.For each of the firing resistors there exists a similar serialcorrection circuit for each positional error, that is, for heaterresistor 811, a horizontal, vertical, and rotational position correctioncircuit is available to modify the timing of the firing pulse outputfrom droplet firing controller 115. Likewise for heater resistor 813,similar circuits exist. While the preferred embodiment utilizes thecorrection circuits as shown, it is obvious that the circuits could berepositioned in their sequence of modifying the firing pulse-or that amultiple-purpose circuit could undertake dual or triplicate functions.In some instances, especially when mechanical alignment negates the needfor one of the electronic corrections described herein, one or more ofthe correction circuits may be deleted.

A detailed schematic of the horizontal correction circuit 801 is shownin FIG. 9. A delay in the pulse 913, which is eventually coupled to afiring resistor, is introduced by establishing a pick-off point (anincremental delay) along a shift register 901. A firing pulse 800generated by the droplet firing controller 115 is coupled to the shiftregister 901 and is conventionally clocked to each register of the shiftregister in turn. The print cartridge receives a reference pulse 707from the position controller 113 as described relative to FIG. 7. Thispulse is coupled to the “start up/stop” input port of the up/downcounter −1 of 14 line selector 905 to commence the count and shift a bitin the register of the 1 of 14 line selector. Pulse 713, generated whenthe shadow of opaque stripe 605 occludes the aperture 409, is coupled tothe “start down/stop” input port of the up/down counter of 905 andplaces a counter stop after a number of clock pulses have been appliedto the up/down counter −1 of 14 line selector 905. This results in a bitbeing set in the 1 of 14 line selector corresponding to the time delaybetween time t₁ and t₄. When the firing pulse input into the shiftregister corresponds with the selected line, an “and” gate, for example“and” gate 911, has both inputs active and couples a pulse to gate 909for coupling of a delayed firing pulse 913 out of the horizontalcorrection circuit. It is expected that the pulse 713 will lag the pulse707 by a predetermined number of clock pulses. If pulse 713 occurs toosoon or too late, the delay is changed to accommodate the error andcompensate for the horizontal misalignment. Each cartridge in thecooperatively printing set thus has its delay offset so that all produceprinted ink droplets at precisely the correct time relative to its trueposition in the print cartridge carriage. In this way horizontal (“X”)alignment between the individual cartridges is established with a highdegree of precision.

Similarly vertical offset is established by detecting the timing ofvertically differentiable reference indicia. The objective is to selectthe best contiguous set of orifices to be used to print a character orimage. In the preferred embodiment where fifty orifices are used toprint, fifty-four orifices are actually available. With a perfectvertical alignment, the top and bottom two orifices will remain unusedwhile the centered fifty are selected for printing. If the printcartridge is positioned low relative to nominal, the top one orifice andbottom three orifices will remain unused while the fifty orificesbetween will become the selected set.

Referring now to FIG. 10, a more detailed schematic of the verticalcorrection circuit 803 is shown. Pulses 713 and 715, generated as theshadows of opaque stripes 605 and 603 sequentially occlude aperture 409,are coupled to an up/down counter −1 of 8 line selector 1101. Pulse 713is first delayed by a predetermined time coresponding to the expectedtime delay between t₁ and t₄ and then applied to the “start up/stop”port of the up/down counter and pulse 715 is applied to the “startdown/stop” port of the up down counter. If pulses 713 and 715 are notessentially coincident after 713 is delayed by delay 1002, a line otherthan the line corresponding to a zero firing pulse delay is selected inthe 1 of 8 line selector. A 1 of 7 line selection may be made byselector 1003. When the line select is made, for example a selection ofa line corresponding to a delay of −1 clock cycle, register −1 is theline selected. The state of register 01 is coupled to one input of an“and” gate 1005. The other input is coupled to the firing pulsedesignated for the heater resistor corresponding to orifice number (forexample) 50. The register corresponding to no delay is coupled to “and”gate 1007 as is the firing pulse designated for the heater resistorcorresponding to orifice number 49; the register corresponding to adelay of +1 clock cycle is coupled to “and” gate 1009 as is the firingpulse designated for the heater resistor corresponding to orifice number48. Thus when firing pulse 1011, destined for the heater resister oforifice number 50, is input, it is converted to a firing pulse 1013directed to the heater resistor of orifice number 49. The result is thatthe pulses for each resistor are electronically redirected to the firingresistor physically located one orifice beneath the originally selectedorifice. An error in the vertical direction is thus compensated. Aplurality of “and” gates are similarly connected as shown so that eachresistor may have electronically redirected firing pulses as required.

Rotational miss-alignment, that is, a miss-alignment in the θ direction,requires that there be two detection orifices and photosensitivefeatures on the printhead of the cartridge. Assuming the leadingdetection reference indicia (relative to cartridge movement duringalignment) on the printhead face is the feature used to establishhorizontal alignment, a lagging detection feature is then use toestablish degree of cartridge orifice rotation. The rotationalcorrection circuit 805 of the preferred embodiment is based upon knowingthe intended horizontal separation of the two detection features. A 1 of8 line selector −up/down pulse counter 1101, as shown in FIG. 11, isstarted when the first alignment pulse 709 (from the photosensitive areaassociated with alignment orifice 407) is input to the “start up/stop”input to start the pulse counter after being delayed by delay 1103 forthe expected delay time t₄−t₂ The counter is stopped when the secondalignment pulse 713 (from the photosensitive area associated withalignment orifice 409 produced by the same vertically extended referenceindicia) is coupled to the “start down/stop” input of the counter. Sincethe datum in the preferred embodiment is established at the alignmentaperture 409, rotational errors are defined as rotation about thisdatum. Orifices which are disposed furthest from the datum experiencethe greatest amount of deviation from the desired position; orificesdisposed closest to the datum experience the least amount of deviation.Also, the most troublesome deviation occurs in the vertical directionrather than in the horizontal. Accordingly, the implementation in thepreferred embodiment selectively corrects the vertical deviation. Thoseheater resistors associated with orifices furthest from the datumorifice are caused to experience a correction in the vertical directionwhile those closest to the datum are not. The line select is determinedby the difference between the delayed pulse 709 and the pulse 713. Asshown for the preferred embodiment in FIG. 11, the line select is set bythe time difference in pulses 709 and 713. The state of the line selectis anded with the input pulse 804 which is clocked through the shiftregister 1106 so that, for the heater resistors corresponding to theorifices furthest away from the datum, when the pulse 804 reaches theregister which is coupled to the “and” gate (for example, “and” gate1105) which is connected to the line select with the active state, aheater resistor firing pulse 1121 is applied to the heater resistor.

For those heater resistors which are associated with orifices closer tothe datum, the amount of time delay allowed is compressed by couplingtwo or more of the line select registers together with an “or” gate.This is shown in the schematic of FIG. 12. In the preferred embodiment,line select registers corresponding to time increments of −3 and −2clock pulses are coupled to an “or” gate 1203 and then to an “and” gate1205. For line select registers corresponding to −1, 0, and +1 clockpulse delays, the line select register outputs are coupled to an “or”gate 1207 and then to “and” gate 1209. For line select registerscorresponding to +1 and +2 clock pulse delays, the line select registeroutputs are coupled to an “or” gate 1211 and then to “and” gate 1213. Inthis way, a pulse 804 input to the heater resistors of orifices ofintermediate distance from the datum is output as a time shifted pulse1123 with a compressed amount of time shifting. Heater resistors closestto the datum are not shifted in time to correct for rotational errors inthe positioning of the print cartridge.

In an alternative embodiment a more sophisticated mapping scheme cancompensate vertical as well as rotational errors. Also, if the cartridgeis capable of printing gray scale, a gray scale level adjustment couldbe made at this time. Furthermore, a more sophisticated rotationcorrection scheme would compensate for uniform change in orifice platesize due to manufacturing tolerance or change in temperature. In thisalternative, the distance between the two detectors on the printheadface is essentially measured by using both the horizontal and verticalcrossing timing information. This information is ratioed with thehorizontal offset to produce a better estimate of θ error and thereforea more robust rotation correction, independent of uniform orifice sizechanges.

Alignment of the cartridge in the preferred embodiment is keyed to theconventional servicing cycle of the print cartridges in a printer. Aalignment cycle would also be run at printer turn on. Alternativealignment algorithms for the cartridges could take place as often asonce per printing pass just prior to beginning the print swath. Also adetection of a change printhead temperature, a new page, or simply thepassage of time or number of print swaths completed could also be usedto determine when to perform an alignment cycle.

What is claimed is:
 1. A method for dynamically compensatingmisalignment of a printhead employing an ink drop ejection apparatus toexpel ink in a controlled manner to effect printing of ink dotsdeposited sequentially on a medium by a printer having an alignmentmechanism, comprising the steps of: determining a need for a firstalignment cycle; following said need determination, moving the printheadpast the alignment mechanism, which comprises at least two spaced apartreference indicia; detecting a plurality of timing pulses; detectingsaid passing of a first of said at least two reference indicia;detecting said passing of a second of said at least two referenceindicia; determining a period of time related to said detection of aplurality of timing pulses between said detection of said firstreference indicia passage and said detection of said second referenceindicia passage; creating a delay time related to said determined periodof time; and delaying energization by said created delay time of atleast a portion of the ink drop election apparatus for at least twosequential ink expulsion events and until a second alignment cycle isneeded.
 2. The method in accordance with claim 1 further comprising thestep of generating said plurality of timing pulses from an encodedposition of the printhead relative to the printer.
 3. The method inaccordance with claim 1 further comprising the step of determining aneed for said first alignment cycle further comprises the step ofdetecting a printhead service requirement.
 4. The method in accordancewith claim 1 further comprising the step of determining a need for saidfirst alignment cycle further comprises the step of detecting anapplication of electrical power to the printer.
 5. The method inaccordance with claim 1 further comprising the step of determining aneed for said first alignment cycle further comprises the step ofdetecting a temperature change increment in the printhead.
 6. The methodin accordance with claim 1 further comprising the step of determining aneed for said second alignment cycle further comprises the step ofdetecting a new media sheet available for printing.
 7. The method inaccordance with claim 1 further comprising the step of determining aneed for said second alignment cycle further comprises the step ofdetecting a passage of a predetermined period of time unrelated to saidtiming pulses.
 8. The method in accordance with claim 1 furthercomprising the step of determining a need for said second alignmentcycle further comprises the step of detecting a temperature changeincrement in the printhead.
 9. The method in accordance with claim 1further comprising the step of determining a need for said secondalignment cycle further comprises the step of detecting a printheadservice requirement.